Lightweight paper and process for producing same

ABSTRACT

A lightweight paper having a superior opacity and printing opacity is provided, which lightweight paper comprises dry pulp, 0.015 to 1.2% by weight based on the pulp of an alumina polymer and 0.5 to 30% by weight based on the pulp of a coaggregate formed from agglomerated particles of urea-formal-dehyde polymer (A) and agglomerated particles of hydrated silicic acid (B) in a ratio of (A):(B) of 5:95 to 95:5.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a lightweight paper comprising a coaggregateformed from agglomerated particles of urea-formaldehyde polymer andagglomerated particles of hydrated silicic acid, and a process forproducing the same.

2. Description of Related Art

Making printing paper and newspaper roll, lightweight has been conductedby making paper thinner, but there may occur an obstacle of theso-called print through. Such print through includes show through due toreduction in the opacity of paper (hereinafter referred to as "opacity")and the so-called strike through that printed ink runs through paper andit is seen from its back surface (hereinafter the extent to which it isprevented will be referred to "printing opacity"). Either of the abovephysical properties refer to a phenomenon that print on the back surfaceis seen from the front surface to make reading of print on the frontsurface difficult; hence such physical properties are most important atthe time of making lightweight paper.

Agglomerated particles of hydrated silicic acid are called whilte carbonand have been used for preventing the print through (e.g. U.S. Pat. Nos.4,132,806, 4,161,455, 4,157,920 and 4,202,813) but they have almost noeffect of improving the opacity, and moreover, the percentage fixationthereof relative to pulp at the time of paper-making is so weak that theretention thereof is low.

Further, agglomerated particles of urea-formaldehyde polymer have beenused for improving the brightness of paper and the opacity (e.g. seeU.S. Pat. No. 3,909,348), but the effect of improving the printingopacity is still insufficient although the agglomerates ofurea-formaldehyde polymer has a stronger fixability onto pulp than thatof white carbon and hence the percentage retention is higher.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a lightweight paperhaving superior physical properties mainly brought about by the functionof highly improving its printing opacity, while retaining the functionof improving the opacity and the high fixability, each of agglomeratedparticles of urea-formaldehyde polymer.

The present invention in a first aspect resides in a lightweight papercomprising dry pulp, 0.015 to 1.2% by weight based on the pulp of analumina polymer and 0.5 to 30% by weight based on the pulp of acoaggregate formed from agglomerated particles of urea-formaldehydepolymer (A) and agglomerated particles of hydrated silicic acid (B) in aratio of (A):(B) of 5:95 to 95:5.

The present invention in a second aspect resides in a process forproducing a lightweight paper, which process comprises adding into apulp slurry, an aluminium salt in a quantity required for forming 0.015to 1.2% by weight based on dry pulp, of an alumina polymer and furtheradding agglomerated particles of urea-formaldehyde polymer (A) andagglomerated particles of hydrated silicic acid (B) in quantitiesrequired for giving a weight ratio of (A):(B) of 5:95 to 95:5 and 0.5 to30% by weight based on dry pulp, of (A) and (B), followed bypaper-making.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The agglomerated particles of urea-formaldehyde polymer used in thepresent invention are preferred to have an average particle diameter of0.1 to 0.5μ and an average diameter of the agglomerated particles of 1to 15μ. If the average particle diameter is less than 0.1μ, theresulting agglomerated particles have a low strength, so that when theyare used in the process of paper-making, they collapse due to thepressure applied to the paper during the process of paper-making to makeinsufficient the objective function of improving the printing opacity.On the other hand, if the average particle diameter exceeds 0.5μ, theresulting coaggregate of associated particles of urea-formaldehydepolymer, white carbon and the alumina polymer (hereinafter abbreviatedto "teraggregate") has a low percentage retention in the resultingpaper, and also converted paper after printing has a low opacity. Themore preferable average particle diameter is in the range of 0.12 to0.3μ and such a range is usually employed. Further if the averagediameter of the agglomerated particles is less than 1μ, the resultingteraggregate has a low retention in the resulting paper, so thatconverted paper after printing has a low opacity. If the averagediameter of the agglomerated particles exceeds 15μ, resultingteraggregate has a high percentage fixation to paper, but it has a lowdispensibility in paper so that the printing opacity, the opacity, etc.lower. Further, the more preferable range of the average diameter ofagglomerated particles is in the range of 2 to 10μ and such a range isusually employed.

The above-mentioned agglomerated particles of urea-formaldehyde polymerare easily prepared according to a known, optional method. For example,the agglomerated particles are obtained by way of either a one-stepprocess or a two-step process, and in either of the processes, thepolymer particles are prepared so as to have an optional ratio by mol ofurea to formaldehyde. In more detail, in the case of the two-stepprocess, a water-soluble urea-formaldehyde condensate is firstly formed,followed by curing the water-soluble, initial condensate in the presenceof a curing catalyst at an elevated temperature to thereby formagglomerated particles of the polymer.

Further, in the case of a one-step process, all components and additivesused for the reaction are first added, and the reaction proceedsdirectly until the agglomerated particles of the polymer are formed. Inthe respective cases, the resulting agglomerated particles ofurea-formaldehyde polymer are neutralized and washed with water toremove free formaldehyde, or before the neutralization, urea, ammonia,an ammonium salt, sulfurous acid or a sulfite salt is added and reactedto remove free formaldehyde, followed by neutralization, and thereafterfiltration or centrifugal separation to recover agglomerated particlesof urea-formaldehyde polymer in the form of a cake, or drying theparticles in a conventional manner such as spray drying, further airdrying and other contact and convectional drying, etc. In the case wherethe agglomerated particles of urea-formaldehyde polymer are used in theform of a cake or used in the form of a slurry by redispersing the cake,in water milling is carried out before the agglomerated particles aremade into a cake-form, to adjust the average diameter of theagglomerated particles preferably to 2 to 10μ. Further in the case wherethe agglomerated particles of urea-formaldehyde polymer are obtained ina dry state, milling is carried out after drying to adjust the averagediameter of the agglomerated particles preferably to 2 to 10μ. Thefiltrate obtained by the above-mentioned filtration or centrifugalseparation is used as water for raw materials or water for adjustment atthe prior step.

The curing catalyst usable in the preparation of the agglomeratedparticles of urea-formaldehyde polymer includes acidic catalysts, forexample, a mineral acid such as sulfuric acid, phosphoric acid,hydrochloric acid, nitric acid, organic acids having medium PK valuesless than 4 such as formic acid, oxalic acid, maleic acid, succinicacid, chloracetic acid or the like acid. Further, sulfamic acid or awater soluble ammonium hydrogensulfate of the formula RNH₃ SO₄ H(wherein R represents hydrogen, alkyl group, cycloalkyl group,hydroxyalkyl group, aralkyl group, aryl group, etc.) may be also used.The water-soluble ammonium hydrogen sulfate refers to methylammoniumhydrogensulfate, ethylammonium hydrogensulfate, hydroxyethylammoniumhydrogensulfate, phenylammonium hydrogensulfate, benzylammoniumhydrogensulfate, etc.

In order to form particles having a preferable particle diameter in theproduction of the agglomerated particles of urea-formaldehyde polymer,it is advantageous to add a water-soluble, organic high molecular weightpolymer having a function of a protective colloid, to a water-soluble,initial urea-formaldehyde condensate in advance of forming particles ofa mixed aqueous solution of urea and formaldehyde. The water-soluble,organic high molecular weight polymer having a function of a protectivecolloid referred to herein means natural substances such as starch,gelatin, hide glue, tragacanth gum, agar, acacia gum, etc., modifiednatural substances such as alkali metal salts e.g. sodium salts,potassium salts, etc. of carboxymethyl cellulose, carboxyethylcellulose, etc., alkali metal salts of methyl cellulose, ethylcellulose, β-hydroxyethyl cellulose, alginic acid, etc., polyvinylalcohol, polyvinyl pyrohidone, polyacrylic acid, polymethacrylic acidand alkali metal salts thereof, copolymers of maleic acid with styreneor butylene and salts thereof, salts of homopolymer or copolymers ofvinyl pyridine, etc. The quantity of the protective colloid agent usedis generally in the range of about 0.1 to 10% by weight (hereinafter %means % by weight), preferably 0.5 to 5% based on the weight of urea andformaldehyde, although it depends on its kind.

Next, an advantageous process for producing the agglomerated particlesof urea-formaldehyde will be described in detail.

Usually, a water-soluble, initial urea-formaldehyde condensate in amolar ratio of 1:1 to 1:2 is used as an intermediate raw material, whichis obtained by reacting an aqueous solution of urea, formaldehyde andadditives having a total concentration of about 20 to 75%, at atemperature of about 30° to 100° C., at a pH of about 5 to 9 and for 10minutes to 4 hours. As the protective colloid agent, polyvinyl alcoholor sodium salt of carboxymethyl cellulose is used and this may be addedat an optional time during the preparation of the water-soluble, initialurea-formaldehyde condensate. As the subsequent step, to the initialcondensate containing the protective colloid agent is added a solutionof sulfuric acid or sulfamic acid at a temperature of room temperatureto about 100° C., with stirring, until gelation occurs, followed byroughly grinding the resulting agglomerated particles into those havinga diameter of 1 to 2 mm by means of a pelletizer or a hammer mill,thereafter adding water with stirring to obtain a slurry having aconcentration of the agglomerated particles of urea-formaldehyde polymerof 5 to 10%, successively neutralizing the resulting material withaqueous ammonia or an aqueous solution of an alkali such as sodiumhydroxide, milling the resulting material into agglomerated particleshaving a diameter of 2 to 10μ by means of a mill and dehydrating bymeans of a filtration-dehydrator to obtain cake-form, agglomeratedparticles of urea-formaldehyde polymer.

The agglomerated particles of hydrated silicic acid used in combinationwith the above agglomerated particles and others are preferred to have aBET specific surface area (measured according to Brunauer Emette andTeller's method) of 100 to 300 m² /g. If the BET specific surface areais less than 100 m² /g, converted paper obtained using the teraggregateformed from the agglomerated particles of hydrated silicic acid, theagglomerated particles of urea-formaldehyde polymer and the aluminapolymer is insufficient in the function of improving the printingopacity. On the other hand, if the BET specific surface area exceeds 300m² /g, the association strength of the agglomerated particles ofhydrated silicic acid is so weak that when the agglomerated particlesare used at the step of paper-making, the agglomerated particlescollapse due to the pressure applied to the paper during the step ofpaper-making, and as a result, the aimed function of improving theprinting opacity is insufficient. The more preferred BET specificsurface area is in the range of 150 to 250 m² /g.

The agglomerated particles of silicic acid having such surface areas areeasily prepared according to a known method. In general, theagglomerated particles can be obtained by reacting an alkali silicateand a mineral acid as well as a salt in an aqueous solution, and usuallyan aqueous solution of a mineral acid prepared so as to have aconcentration of 2 to 40 g/100 cc is added to an aqueous solution of analkali silicate prepared in advance so as to have a concentration of 2to 9.5 g/100 cc as calculated in terms of silica. The addition reactionis carried out at a temperature of 65° C. or higher. As the method ofadding the aqueous solution of a mineral acid, there are a method ofadding it continuously and a method of adding it in divided portions,but in the former case of continuous addition, a product having a morestable quality is liable to be obtained, and yet the operation iseasier. Usually, in the case of adding it continuously, it is preferredto complete the addition at a time of 50 minutes or shorter. The BETspecific surface area of the agglomerated particles of hydrated silicicacid depends mainly upon the addition rate of the mineral acid i.e. theformation rate of particles of hydrated silicic acid. There is atendency that the lower the addition rate, the smaller the BET specificsurface area, while the higher the addition rate, the greater the BETspecific surface area.

The alkali silicate as the raw material of the above agglomeratedparticles of hydrated silicic acid may be those which can be expressedby SiO₂ /alkali (molar ratio), and currently commercially availablewater glass may be used at it is. Further, as the mineral acid used asits raw material, mineral acids such as sulfuric acid, hydrochloricacid, nitric acid, etc. are usable, but sulfuric acid is suitable in theaspect of its effect upon the papermaking step.

In the preparation of the agglomerated particles of hydrated silicicacid, various techniques already employed for the preparation of thehydrated silicic acid referred to as white carbon such as techniques ofadding sodium sulfate, sodium chloride, etc. or successively raising thereaction temperature are applicable if necessary.

The agglomerated particles of hydrated silicic acid are obtained in theform of slurry as described above, but if necessary, the resultingslurry may be subjected to centrifugal dehydration orfiltration-dehydration to obtain a cake-form material, or this cake-formmaterial may be further dried to obtain a powdery material. When such acake-form or powdery material is used, it is returned to a slurry byadding water.

In the practice of the present invention, it is necessary to add toaluminium salt for forming 0.015 to 1.2% based on dry pulp, of analumina polymer, into a pulp slurry. If the proportion of the aluminapolymer is less than 0.015% or exceeds 1.2%, the teraggregate formedfrom the alumina polymer, the agglomerated particles of ureaformaldehydepolymer (A) and the agglomerated particles of hydrated silicic acid (B)has a low percentage retention thereof in paper, so that it isimpossible to acquire as sufficient opacity of converted paper afterprinting and opacity and an improved brightness. The quantity of thealumina polymer formed is preferred to be in the range of 0.04 to 0.75%. As the aluminium salt used for forming the alumina polymer, aluminiumsulfate, aluminum chloride, sodium aluminate, etc. are usable, but useof aluminum sulfate is preferred in view of stabilized formation of thealumina polymer. Aluminum sulfate is expressed by the formula Al₂ (SO₄)₃and this is hydrolyzed in pulp slurry to form an aluminum hydroxidepolymer having cations. The quantity by weight of this aluminumhydroxide polymer prepared refers to the quantity calculated in terms ofAl₂ O₃ from the total quantity of the aluminum salt added and thealuminum salt contained in white water circulated.

The agglomerated particles of urea-formaldehyde polymer (A) and theagglomerated particles of hydrated silicic acid (B) bear a negativepotential in the suspension state in water. It is presumed that thealumina polymer having cationic properties is adsorbed to theseagglomerated particles having a negative potential to afford a cohesionby which the coaggregate of both the agglomerated particles is formed,and at the same time, further reinforce the fixability of theagglomerated particles of urea-formaldehyde polymer (A) onto pulp.

The present invention is characterized by paper-making using theagglomerated particles of urea-formaldehyde polymer (A) and theagglomerated particles of hydrated silicic acid (B) so as to give aratio by weight of (A):(B) of 5:95 to 95:5 and a total weight of (A) and(B) of 0.5 to 30% based on dry pulp. If the proportion of (A) is smallerin a ratio of 5:95, the teraggregate has a low percentage fixationthereof onto paper and the resulting converted paper is insufficient inthe printing opacity, the brightness and the opacity. On the other hand,if the proportion of (B) is smaller in a ratio of 95:5, the resultingteraggregate has a high percentage fixation thereof onto paper, but theresulting converted paper is insufficient in the printing opacity. Theratio of (A):(B) is preferred to be in the range of 20:80 to 80:20.Further the total weight of (A) and (B) is in the range of 0.5 to 30%based on dry pulp. The reason is that if the total weight of (A)+(B) isless than 0.5%, the resulting converted paper is low in the printingopacity, the opacity and the percentage of improvement in thebrightness; hence the object cannot be achieved. On the other hand, ifthe total weight of (A) and (B) exceeds 30%, the resulting convertedpaper has a low strength and the so-called powder drop from paper isobserved to occur; hence it is impossible to sufficiently effect thefunction of paper. Thus the total weight of (A) and (B) is preferred tobe in the range of 1 to 15%.

When the agglomerated particles (A) and (B) are added to pulp slurry,the agglomerated particles of hydrated silicic acid and the agglomeratedparticles of urea-formaldehyde polymer have different percentagesretention, respectively, as described later; hence the quantities ofboth the agglomerated particles are determined so that the ratio byweight of (A):(B) and the total weight of (A) and (B) may fall indefinite ranges, respectively, taking into account the percentagesretention thereof.

When an aluminum salt, the agglomerated particles of urea-formaldehydepolymer (A) and the agglomerated particles of hydrated silicic acid (B)are added to pulp slurry, the addition site thereof may be an optionalsite between the refiner and the fan pump at the paper-making step, andboth the agglomerated particles added are desired to be uniformlydispersed in pulp slurry; thus conventional methods may be employed foragitating and dispersing them. Further, the addition order thereof hasno particular limitation, but it is preferred in the aspect of formingthe coaggregate to mix the agglomerated particles of urea-formaldehydepolymer (A) with the agglomerated particles of hydrated silicic acid (B)in advance, followed by adding the mixture or to add both theagglomerated particles as successively as possible even when they areindividually added. When the aluminum salt and both the agglomeratedparticles are added to pulp slurry, they are each prepared in advanceinto an aqueous solution or slurry having a concentration in which theaddition quantity is easily adjusted, but the concentration is preferredto be as low as possible, from the viewpoint of a uniform dispersibilitythereof in pulp slurry. Usually, pulp slurry having added an aluminumsalt and both the agglomerated particles each prepared so as to have aconcentration of 10% or lower is formed into a thin paper on the wire ofan elongated wire paper machine, a cylindrical wire paper machine or atwin wire paper machine. Usually, by further dehydrating the paper bymeans of a press roll, drying by means of a dryer and finally subjectingit to calender treatment, it is possible to easily produce thelightweight paper of the present invention.

Depending on the use applications of the lightweight paper of thepresent invention, it is preferred to add an additive or an adjustingagent generally and conventionally used, into pulp slurry having addedan aluminum salt and both the agglomerated particles.

It is possible to usually add into pulp slurry, for example, sizingagents such as rosin sizes, synthetic sizes, reactive sizes, paperstrength-reinforcing agents such as those of starch and gums,acrylamides, ureas, melamines, chlorohydrins, etc., waterfiltration-improving agents such as those of ethylene-imines,polyamides, acrylamides, etc., retention aids such as those ofacrylamides, formation-improving adhesives, dyes, detergents, wettingagents, pitch-control agents, etc. It is apparent that the presentinvention also comprises lightweight paper containing such otheradditives.

The above-mentioned lightweight paper of the present invention has afunction of highly improving the printing opacity while retaining thefunction of improving the opacity and a high fixability, and theabove-mentioned process for producing the lightweight paper of thepresent invention is a process which makes easy the production of thelightweight paper having the above-mentioned properties and is effectivefor making paper lightweight.

The present invention will be described by way of examples in moredetail, but it should not be construed to be limited thereto.

EXAMPLE A-1

Into a flask were fed water (20.00 parts by weight) (hereinafter allparts being by weight) and sodium salt of carboxymethyl cellulose(Cellogen F-3H, tradename of product manufactured by Daiichi KogyoSeiyaku Kabushiki Kaisha) (0.325 part), followed by dissolving thesematerials, adding a 37% aqueous solution of formaldehyde (18.24 parts),heating the mixture to 70° C. with stirring, adjusting, at the sametime, the pH to 7.5 with an aqueous solution of NaOH, thereafter addingurea (9 parts), and subjecting the mixture to condensation reaction at70° C. for 2.0 hours to obtain an initial urea-formaldehyde condensationreaction product, cooling this product to about 45° C., and rapidly anduniformly mixing it with a solution obtained by diluting 95% sulfuricacid (0.46 part) with water (15.73 parts). After 10 seconds, thereaction mixture cured and at that time its temperature rose up to 61°C. The solidified material was then kept at about 60° C. for one hour,followed by roughtly grinding it into particles having a size of 1˜2 mmby means of a cutter granulator, adding water (100 parts) to obtain aslurry-form material, neutralizing it with a 20% aqueous solution ofNaOH to make its pH 7.5, milling the resulting slurry by means of amill, and filtration-dehydrating it to obtain a white cake-form material(60.2 parts). A portion thereof was dried by hot air at 105° C. for 2hours and the concentration of the agglomerated particles ofurea-formaldehyde polymer in the cake-form material was measured to give20.1%. Thus, 12.10 parts of the agglomerated particles (A) wereobtained. The agglomerated particles had an average particle diameter of0.2μ as measured by means of the photograph of an electron microscope,and also had an average diameter of 5.1μ as measured by means of aCoulter Counter (Model TA II manufactured by Coulter Counter Inc.) andcalculated.

The agglomerated particles of urea-formaldehyde polymer obtained in thisexample are referred to as UF-1.

EXAMPLES A-2 to A-9

Agglomerated particles of urea-formaldehyde polymer (hereinafterreferred to as UF) were obtained in the same manner as in Example A-1except varying the tradename and amount of carboxymethyl cellulosesodium salt used and the clearane of colloid mill (TK Micolloidal Ltype, manufactured by Tokushu Kika Kogyo, Inc.) as set forth in TableX-1. The results obtained are set forth in Table 1.

                  TABLE X-1                                                       ______________________________________                                                                     Clear-                                                                        ance of                                                 Carboxymethyl cellulose                                                                             colloid                                          Exam-        Trade-               Amounts,                                                                              mill,                               ples  UF     names     Makers     Parts  μm                                ______________________________________                                        A-1   UF-1   Cellogen  Daiichi Kogyo                                                                            0.325  35                                                F-3H      Seiyaku K.K.                                           A-2   Uf-2   Hi-Sunlose                                                                              Sanyo Kokusaku                                                                           0.500  35                                                H750      Pulp.Inc.                                              A-3   UF-3   Sunlose   Sanyo Kokusaku                                                                           0.325  35                                                F50MC     Pulp. Inc.                                             A-4   UF-4   Cellogen  As the above                                                                             0.325  20                                                F-3H                                                             A-5   UF-5   Cellogen  As the above                                                                             0.325  50                                                F-3H                                                             A-6   UF-6   Sunlose   Sanyo Kokusaku                                                                           0.500  35                                                PN-01A    Pulp. Inc.                                             A-7   UF-7   Cellogen  As the above                                                                             0.170  35                                                F-3H                                                             A-8   UF-8   Hi-Sunlose                                                                              As the above                                                                             0.325  15                                                H-750                                                            A-9   UF-9   Cellogen  As the above                                                                             0.325  70                                                F-3H                                                             ______________________________________                                    

                  TABLE 1                                                         ______________________________________                                                                 Average                                                              Average  diameter of                                                          particle agglomerated                                                                             Cake                                                      diameter particles  concentration                             Example                                                                              UF       (μ)   (μ)     (%)                                       ______________________________________                                        A-1    UF-1     0.20     5.1        20.1                                      A-2    UF-2     0.12     5.2        19.3                                      A-3    UF-3     0.30     5.1        22.4                                      A-4    UF-4     0.21     2.2        20.3                                      A-5    UF-5     0.20     9.8        20.2                                      A-6    UF-6     0.09     4.9        17.9                                      A-7    UF-7     0.65     6.4        23.4                                      A-8    UF-8     0.15      0.87      19.7                                      A-9    UF-9     0.23     20.6       20.8                                      ______________________________________                                    

EXAMPLE B-1

Sodium silicate (246 cc) having a molar ratio of SiO₂ /Na₂ O of 3.02 anda content of SiO₂ of 19.5 g/100 cc was diluted into 1,200 cc with waterto prepare an aqueous solution of sodium silicate having a concentrationof SiO₂ of 4 g/100 cc, as a raw material. Into a 2 l flask equipped withheating and cooling means, an agitator and a thermometer was placed theabove aqueous solution of sodium silicate as a raw material, followed byraising the temperature up to 90° C. with stirring at 1,000 rpm, andadding 2N sulfuric acid for 40 minutes while heating the mixture so asto keep 90° C., to make the final pH 8.0. The resulting agglomeratedparticles of hydrated silicic acid (B) had a slurry concentration ofabout 3.8%. This slurry was subjected to filtration-dehydration toobtain a cake-form material. A portion thereof was dried in hot air at105° C. for 2 hours and the concentration of the cake was measured togive 21.3%. Further the BET specific surface area was 150 m² /g. Theagglomerated particles of hydrated silicic acid obtained in this Exampleare referred to as WC-1.

EXAMPLES B2 to B5

Agglomerated particles of hydrated silicic acid (hereinafter referred toas WC) were obtained in the same manner as in Example B-1 except varyingthe SiO₂ concentration of the aqueous solution of sodium silicate andthe dropping speed of 2N sulfuric acid as indicated in Table X-2. Theobtained results are set forth in Table 2.

                  TABLE X-2                                                       ______________________________________                                                       S.sub.i O.sub.2 concentration,                                                               Dropping speed of                               Examples                                                                             WC      g/100 cc       2N H.sub.2 SO.sub.4, minutes                    ______________________________________                                        B-1    WC-1    4              40                                              B-2    WC-2    9              13                                              B-3    WC-3    15             60                                              B-4    WC-4    20             60                                              B-5    WC-5    9               1                                              ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                        BET specific   Cake                                           Example                                                                              WC       surface area (m.sup.2 /g)                                                                    concentration (%)                              ______________________________________                                        B-1    WC-1     150            21.3                                           B-2    WC-2     250            21.5                                           B-3    WC-3     100            20.6                                           B-4    WC-4      80            22.8                                           B-5    WC-5     350            21.8                                           ______________________________________                                    

EXAMPLES 1˜5 and COMPARATIVE EXAMPLES 1˜5

Among A-1˜5 and B-1˜3 in the preparation Examples of the agglomeratedparticles of urea-formaldehyde polymer (A) and the agglomeratedparticles of hydrated silicic acid (B), UF-1 and WC-1 the values ofphysical properties of which are close to the central values within thelimited ones were combined to evidence the usefulness of the presentinvention.

An aqueous solution of aluminum sulfate having a concentration of 3.06%as calculated in terms of Al₂ O₃ (the proportion of alumina being about0.31% by weight based on dry pulp) (2.0 parts) was added into a 1% pulpslurry (20.00 parts) having blended therein NB.KP (25 parts), TMP (30parts), RGP (20 parts) and deinked newspaper (25 parts) and having abeating degree (CSF) of 330 ml, followed by agitating the mixture for 2minutes, succesively adding a previously prepared, mixed slurry of UF-1and WC-1 in a ratio by solids weight of the respective associatedparticles of 50:50 and having a concentration of 5% in total (40 parts)(the proportion of both the agglomerated particles being 10% based ondry pulp), and agitating the mixture for 5 minutes to obtain a preparedslurry, then subjecting it to paper-making by means of a TAPPI standardrectangular sheet machine, carrying out press-dehydration, drying theresulting wet paper on a drum dryer having a surface temperature of 110°C., thereafter twice passing it through under a linear pressure of 40kg/cm, and seasoning it in a thermohygrostal at a humidity of 65% and atemperature of 20° C. for 24 hours to obtain a converted paper ofExample 1.

With the converted paper obtained in Example 1, measurements of itsphysical properties such as basic weight, percentage retention,smoothness, bulk density, brightness, opacity, printing opacity, etc.and calculations were carried out.

The results are shown in Table 1.

Converted papers of Examples 2˜5 and Comparative examples 1˜4 wereobtained in the same manner as that in the production of the convertedpaper of Example 1 except that the blending ratio of UF-1 and WC-1 wasvaried. Further, a converted paper of Comparative examples 5 wasobtained in the same manner as that in the process for producing theconverted paper of Example 1 except that the agglomerated particles werenot added. With the converted papers of Examples 2˜5 and Comparativeexamples 1˜5, too, measurements of their physical properties andcalculations were carried out in the same manner as that in the case ofthe converted paper of Example 1. The results are shown in Table 3.

In addition, the methods of measurements of physical properties of paperand calculations are as follows:

The basic weight was measured and calculated by the treatment accordingto JIS (P-8111).

The bulk density was determined by measurement of the thickness of paperaccording to JIS (P-8118) and calculation from an equation of basicweight/thickness×1,000.

The smoothness was measured by menas of Bekk smoothness tester accordingto the method stated in JIS (P-8119) and TAPPI (Standard method T479).

The brightness was measured using a blue filter by means of Huntermeter. Opacity was measured according to JIS (P-8138).

Printing opacity was measured according to a method described in aliterature (Paper and Pulp Art Times, September, 1979, page 1-13).

The percentage retention of UF in the converted paper is calculated bymeasuring the fixed amount of solids of UF added to dry pulp. Inaccordance with the method of TAPPI-T418 SU-72, the nitrogen content inpaper (hereinafter referred to as N₁ %) is measured and the nitrogencontent in UF (hereinafter referred to as N₀ %) is calculated. Forexample, the N₀ in the solids of UF-1 is 28%. The amount of UF in theconverted paper (hereinafter referred to as UF-Y %) is calculated asfollows: ##EQU1##

Then, the percentage retention of UF (hereinafter referred to as UF-X %)is calculated as follows: ##EQU2##

The percentage retention of WC in the converted paper is calculated bymeasuring the content of hydrated silicic acid particles in paper(hereinafter referred to as ω₁ %) in accordance with the method ofTAPPI-T413-ts-66 and the amount of hydrated silicic acid particles addedto dry pulp (hereinafter referred to as ω₀ %) is calculated. ##EQU3##

In the measurement of percentage retention of UF, the above N₁ isdetermined by deducting the nitrogen content contained in the pulpitself.

In the measurement of percentage retention of WC, the above ω₁ % isdetermined by deducting the ash content in converted paper obtained inthe same procedure and conditions except not adding WC.

                                      TABLE 3                                     __________________________________________________________________________                Example                     Comparative example                               1     2    3     4    5     1    2     3   4    5                 __________________________________________________________________________    Amount of alumina                                                                         0.31  0.31 0.31  0.31 0.31 0.31  0.31  0.31                                                                              0.31 0.31              added (%)                                                                     Amount of Agglomera-                                                                      10    10   10    10   10   10    10    10  10   --                ted particles added (%)                                                       UF-1:WC-1 ratio                                                                           50:50 35:65                                                                              20:80 65:35                                                                              80:20                                                                              97:3  3:97  100:0                                                                             0:100                                                                              --                Percentage retention                                                                      57.8/49.5                                                                           57.3/49.2                                                                          58.1/48.8                                                                           57.4/50.4                                                                          56.1/50.7                                                                          55.8/56.3                                                                           60.3/38.9                                                                           55.7/--                                                                           --/38.4                                                                            --                of U/F/WC (%)                                                                 Basic weight (g/cm.sup.2)                                                                 45,1  45,0 45,1  45,2 45,0 45,2  45,1  45,2                                                                              45.1 45.0              Bulk density (g/cm.sup.3)                                                                 0.64  0.63 0.64  0.65 0.64 0.63  0.64  0.63                                                                              0.64 0.65              Smoothness (sec)                                                                          47    45   45    48   44   47    45    48  45   47                Brightness (%)                                                                            56.6  56.1 53.5  56.9 57.3 57.3  50.5  57.4                                                                              50.4 50                Opacity (%) 95.4  95.2 95.0  95.5 95.4 95.5  93.5  95.7                                                                              93.2 89.5              Printing Opacity (%)                                                                      80.9  79.4 78.9  81.2 80.4 77.3  77.4  77.4                                                                              77.5 73.4              __________________________________________________________________________

As apparent from Table 3, the converted papers of Examples 1˜5 havehigher percentages retention than those of the converted papers ofComparative examples 1˜4 although the basic weight, bulk density,smoothness, etc. are the same, so that the former converted papersexhibit higher printing opacities than those of particularly Comparativeexamples 3 and 4 wherein the agglomerated particles are singly added,while retaining higher brightness and opacity.

EXAMPLES 6˜20

Paper-making was carried out in the same manner as in Example 1, fixingthe simultaneous use ratio of the agglomerated particles ofurea-formaldehyde polymer (A) to the agglomerated particles of hydratedsilicic acid (B) to 60:40 and also fixing the total quantity of theseagglomerated particles added, to 10%, except that the other factors werevaried, followed by measurement of the physical properties of theresulting converted papers and calculation. The results are shown inTable 4.

As apparent from Table 4, the converted papers obtained under theconditions of the present inventions are improved in any of brightness,opacity and printing opacity.

                                      TABLE 4                                     __________________________________________________________________________               Example                                                                       6  7  8  9  10 11 12 13 14 15 16 17 18 19 20                       __________________________________________________________________________    UF                                                                            UF-No.     1  2  3  4  5  1  1  6  7  8  9  1  1  1  1                        Average particle                                                                         0.20                                                                             0.12                                                                             0.30                                                                             0.21                                                                             0.20                                                                             0.20                                                                             0.20                                                                             0.09                                                                             0.65                                                                             0.15                                                                             0.23                                                                             0.20                                                                             0.20                                                                             0.20                                                                             0.20                     diameter (μ)                                                               Average diameter                                                                         5.1                                                                              5.2                                                                              5.1                                                                              2.2                                                                              9.8                                                                              5.1                                                                              5.1                                                                              4.9                                                                              6.4                                                                              0.87                                                                             20.6                                                                             5.1                                                                              5.1                                                                              5.1                                                                              5.1                      of Agglomerated                                                               particles (μ)                                                              WC                                                                            WC-No.     1  1  1  1  1  2  3  1  1  1  1  4  5  1  1                        BET specific                                                                             150                                                                              150                                                                              150                                                                              150                                                                              150                                                                              250                                                                              100                                                                              150                                                                              150                                                                              150                                                                              150                                                                              80 350                                                                              150                                                                              150                      surface area                                                                  (m.sup.2 /g)                                                                  Simultaneous use                                                                         60/                                                                              60/                                                                              60/                                                                              60/                                                                              60/                                                                              60/                                                                              60/                                                                              60/                                                                              60/                                                                              60/                                                                              60/                                                                              60/                                                                              60/                                                                              60/                                                                              60/                      ratio of UF:WC                                                                           40 40 40 40 40 40 40 40 40 40 40 40 40 40 40                       Total amount of                                                                          10 10 10 10 10 10 10 10 10 10 10 10 10 10 10                       UF and WC added (%)                                                           Amount of alumina                                                                        0.31                                                                             0.31                                                                             0.31                                                                             0.31                                                                             0.31                                                                             0.31                                                                             0.31                                                                             0.31                                                                             0.31                                                                             0.31                                                                             0.31                                                                             0.31                                                                             0.31                                                                             0.005                                                                            0.90                     added (%)                                                                     Percentage retention                                                                     57.2/                                                                            57.4/                                                                            57.6/                                                                            56.4/                                                                            58.8/                                                                            57.5/                                                                            57.6/                                                                            57.3/                                                                            58.1/                                                                            49.9/                                                                            60.8/                                                                            57.5/                                                                            57.8/                                                                            57.4/                                                                            56.8/                    of UF/WC (%)                                                                             50.5                                                                             50.8                                                                             50.9                                                                             50.4                                                                             51.2                                                                             51.8                                                                             50.9                                                                             50.6                                                                             50.8                                                                             50.3                                                                             51.9                                                                             50.7                                                                             51.8                                                                             40.9                                                                             40.7                     Basic weight (g/m.sup.2)                                                                 45.2                                                                             45.0                                                                             45.2                                                                             45.1                                                                             45.2                                                                             45.0                                                                             45.1                                                                             45.2                                                                             45.2                                                                             45.0                                                                             45.1                                                                             45.1                                                                             45.0                                                                             45.2                                                                             45.1                     Bulk density (g/cm.sup.3)                                                                0.62                                                                             0.64                                                                             0.64                                                                             0.62                                                                             0.63                                                                             0.62                                                                             0.64                                                                             0.63                                                                             0.62                                                                             0.64                                                                             0.63                                                                             0.63                                                                             0.62                                                                             0.64                                                                             0.62                     Smoothness (sec)                                                                         46 44 47 44 46 45 46 47 44 46 47 45 45 47 46                       Brightness (%)                                                                           56.7                                                                             56.2                                                                             56.1                                                                             56.8                                                                             56.7                                                                             56.5                                                                             56.6                                                                             51.9                                                                             52.1                                                                             53.2                                                                             53.5                                                                             56.5                                                                             56.6                                                                             56.5                                                                             55.1                     Opacity (%)                                                                              95.3                                                                             95.1                                                                             94.9                                                                             95.1                                                                             95.4                                                                             95.3                                                                             95.4                                                                             93.8                                                                             94.1                                                                             94.5                                                                             94.4                                                                             95.2                                                                             95.4                                                                             95.5                                                                             94.8                     Printing Opacity (%)                                                                     81.0                                                                             82.1                                                                             80.3                                                                             80.1                                                                             80.2                                                                             81.5                                                                             80.6                                                                             75.8                                                                             74.3                                                                             75.9                                                                             76.3                                                                             75.4                                                                             76.4                                                                             76.5                                                                             76.8                     __________________________________________________________________________

EXAMPLES 21˜29 AND COMPARATIVE EXAMPLE 6

The instant Examples and Comparative example illustrate making paperlightweight.

In Examples 21˜29, converted papers were obtained under the sameconditions and process as those in Example 1 except that under thepaper-making conditioned of Example 1, aluminum sulfate was added topulp slurry, followed by further adding a maleic rosin size in aquantity of 0.15% by weight based on dry pulp; the quantities of UF-1and WC-1 added were varied and the simultaneous use ratio thereof waschanged to 60:40; and the basic weight of paper was varied. Further, incomparative example 6, converted paper was obtained under the sameconditions and process as those in Examples 21˜29 except that UF-1, WC-1and maleic rosin were not added and the basic weight of made paper was50 g/m². The resulting respective converted papers were subjected to thesame evaluation of physical properties as that carried out in Example 1,and further, a test of water absorption properties was carried out. Thepaper-making conditions, measurements of physical properties andcalculation results of these converted papers are shown in Table 5.

In addition, the test of water absorption properties was carried out bydropwise adding distilled water (0.04 ml) onto paper surface by means ofa syringe and measuring the time by which water drops were absorbed andextinct on the paper surface.

As apparent from Table 5, by the simultaneous use of the agglomeratedparticles of urea-formaldehyde polymer and the agglomerated particles ofhydrated silicic acid, it is possible to make paper lightweight, whileretaining the opacity and the printing opacity, that is, withoutreducing the percentage print through.

                                      TABLE 5                                     __________________________________________________________________________                                                     Comp.                                     Example                             ex.                                       21  22  23  24  25  26  27  28  29  6                            __________________________________________________________________________    Amount of Agglomerated                                                                     2   2   2   5   5   5    10 10  10  --                           particles added (%)                                                           Simultaneous use of                                                                        60/40                                                                             60/40                                                                             60/40                                                                             60/40                                                                             60/40                                                                             60/40                                                                             60/40                                                                             60/40                                                                             60/40                                                                             --                           U/F-1:WC-1                                                                    Amount of alumina                                                                          0.31                                                                              0.31                                                                              0.31                                                                              0.31                                                                              0.31                                                                              0.31                                                                              0.31                                                                              0.31                                                                              0.31                                                                              0.31                         added (%)                                                                     Basic weight (g/m.sup.2)                                                                   46  44  42  46  44  42  46  44  42  50                           Bulk density (g/cm.sup.3)                                                                  0.62                                                                              0.64                                                                              0.64                                                                              0.63                                                                              0.65                                                                              0.64                                                                              0.63                                                                              0.64                                                                              0.65                                                                              0.63                         Smoothness (sec)                                                                           48  46  45  47  45  46  46  45  44  47                           Brightness (%)                                                                             51.5                                                                              51.4                                                                              51.6                                                                              53.3                                                                              53.4                                                                              53.3                                                                              56.8                                                                              56.7                                                                              56.8                                                                              50.1                         Opacity (%)  92.7                                                                              91.4                                                                              90.1                                                                              93.6                                                                              92.5                                                                              91.2                                                                              95.9                                                                              94.9                                                                              93.5                                                                              92.5                         Printing Opacity (%)                                                                       76.2                                                                              73.1                                                                              70.8                                                                              78.4                                                                              76.1                                                                              73.8                                                                              80.7                                                                              78.6                                                                              76.9                                                                              76.1                         Water absorption                                                                           10  9   10  8   9   9   6   6   5   6                            degree (sec)                                                                  __________________________________________________________________________

What is claimed is:
 1. A process for producing a lightweight paper,which process comprises adding into a pulp slurry, an aluminum salt in aquantity required for forming 0.015 to 1.2% by weight based on dry pulp,of an alumina polymer and further adding a mixture of agglomeratedparticles of urea-formaldehyde polymer (A) and agglomerated particles ofhydrated silicic acid (B) thereby forming a teraggregate of(A)/(B)/alumina polymer, said urea-formaldehyde particles having anaverage particle diameter of 0.1 to 0.5μ and an average agglomeratedparticles diameter of 1 to 15μ, and said silica acid particles having aBET specific surface area of 100 to 300 m/g, in quantities required forgiving a ratio of (A):(B) in the range of 5:95 to 95:5 and 0.5 to 30% byweight based on dry pulp, or (A) and (B), followed by paper-making.
 2. Aprocess according to claim 1 wherein the ratio of (A):(B) is in therange of 20:80 to 80:20 and the total weight based on the pulp, of (A)and (B) is in the range of 1 to 15% by weight.
 3. A process according toclaim 1 wherein said alumina polymer is formed from aluminum sulfate,aluminum chloride or sodium aluminate.
 4. A process according to claim 1wherein the quantity of said alumina polymer is in the range of 0.04 to0.75% by weight based on said pulp.
 5. A lightweight paper obtained bythe process according to claim 1.